Many chemical and biological measurements need to be performed in locations outside of a fully equipped analytical facility. This requires systems that are portable and miniaturized so that they can be transported to locations where rapid test response is required for process or water quality monitoring, or they can be deployed in a medical environment to provide rapid test results for certain biological or biochemical species of interest. These chemical and biological analyses can be performed individually using single tests with post-test optimization to enhance the quality or accuracy of the results, but this serial approach has inherent flaws because multidimensional interactions are difficult to fully compensate using a serial approach. Additionally, this approach can be time consuming and can produce erroneous results. Introducing operator or system errors when tests are performed on different platforms or at different times further complicates this system. The best way to overcome this limitation is to perform all the desired measurements simultaneously on the same platform, but current state of the art does not provide a fully integrated platform for such measurements.
Current electrochemical array technology allows operators to perform an arrayed test at a single time, but this is limited to those materials that respond to an electrochemical stimulus. This usually involves measurement techniques like anodic stripping voltametry or cyclic voltametry methods, or incorporates chemically responsive material into an electrochemical detector, e.g., an ion-specific electrode (ISE). This system, although productive for some systems is limited by many of the common limitations of electrochemical systems, e.g., systematic problems at low and high ionic strength that effect electrochemical potentials. Additionally, some of these systems can suffer from serious cross-reactivities or interferences, e.g., the cross-reactivity of common oxyanions or small cations like sodium, lithium, and potassium.
There are other test platforms that could provide small-scale array measurements that are based on optical or spectral measurements. These can be optical detection from multi-flow wet chemical analysis, or they can be portable versions of classic laboratory measures, e.g., portable atomic absorption spectrometer units. These systems are often limited by mechanics required for fluid flow and maintenance, or from cumbersome equipment like portable atomic absorption spectrometer systems that, although theoretically transportable, have proven less mobile in practice. There is also mention of miniaturizing additional lab systems like Inductively Coupled Plasma—Atomic Emission Spectroscopy or Mass Spectroscopy, but these methods are difficult to adapt into portable handheld, or field deployable systems.
A proposed alternative is to use an optical platform based on well-characterized, chemical responses of optical sensor films. Such a system uses solid, chemically responsive films that respond to analyte concentrations by changing their absorbance values at an optimized wavelength. This platform can be extended to incorporate sensor test elements for all known interfering or cross-reactive species for a particular test matrix, as well as account for test limitations at the extremes of the test sample conditions, e.g., high and low ionic strength as well as high and low buffer strengths. This system has the added benefit of providing a small test platform that can be formed into an array specifically designed to measure test elements that require specific deconvolution analysis.
Optical chemical sensors also fall into two general classes, reversible and irreversible. Fully reversible sensors equilibrate rapidly to the concentration of the target analyte in the test fluid and their response changes when the analyte concentration changes. Examples of reversible sensors are polymer film pH sensors and ion selective optodes (ISO). In contrast, an irreversible sensor will continue to respond to the analyte in the test fluid until the responsive reagent in the sensor has been exhausted, i.e., the total amount of analyte available to the sensor rather than the analyte concentration in the sample. Many non-ISO type sensors belong to this category.
Since the reagent in the reversible sensors is at chemical equilibrium with the analyte in the sample, the exposure of the sensor film to the sample alters the analyte concentration if the sample volume is finite. This requires that the reversible sensor films be exposed to either a large excess amount of sample volume or a given amount of sample volume. In the latter case, a correction can be made to reduce errors due to the finite volume effect. Similarly, the irreversible sensor requires sample volume control so that the sensor response reflects the analyte concentration in the controlled volume of the test fluid.
A sensor array designed for quantitative analysis may not yield satisfactory results by just immersing the array element into a liquid sample because the above mention reasons. For a sensor array consisting of both reversible and irreversible sensors, the sample volume that each sensor region is exposed to has to be controlled. Moreover, volume regulation also helps prevent sensor-to-sensor cross contamination. In this invention, sensor film compositions are designed to be at their optimized performance when they are exposed to fixed sample volumes.
Optical sensor arrays that are composed of irreversible sensors, or a combination of irreversible and reversible sensors, must have some form of fluidic control that delivers a controlled volume of test fluid to each sensor element. Most systems available today use some form of pump or mechanical multi-addition system to deliver these controlled volumes, e.g., robotic addition to multi-well plates. These systems are often cumbersome and require mechanical and electrical components that are rarely field robust and suitable for remote testing in harsh environments.
In recent years, capillary effect has been exploited for fluidic designs. One drawback associated with this passive mechanism is that it relies on the use of absorbent or wicking materials. This makes it difficult to fabricate a device to deliver a small amount of sample to a large number of locations. Instead, the absorbent material is an integrated part of sensing or reaction matrix. Liquid delivered to the site result in only wetting the materials within the matrix. For sensor array applications, dosing a given amount of liquid sample to multiple locations is more desirable.
Even though a large number of publications and patents have been devoted to the development of sensor methods, reagents, and equipment to replace the traditional wet chemistry methods, a need remains for an economical and convenient field deployable sensor system for simultaneous detection of multiple analytes.